Introduction

Any energy engineer knows firsthand what a transformer is and how it works. What is needed for reliable operation of a transformer? One of the criteria is transformer oil. This work will help you learn more about transformer oil. She will tell you not only about the oil itself, but also about the methods of drying it, as well as the technical requirements for operation.

Transformer oil

Physical indicators

The density of transformer oils ranges from 800-890 kg/m3 and depends on its chemical composition. The more polycyclic aromatic and naphthenic hydrocarbons in the oil, the higher its density. The molecular weight of transformer oils ranges from 230-330 and depends on their fractional and chemical composition. With a similar fractional composition, the more aromatic hydrocarbons in the oil, the lower the molecular weight and density, that is, as the oil becomes more refined, the density decreases and its molecular weight increases.

The molecular weight of oils is determined by ebullioscopic or cryoscopic methods. Both methods are based on the laws of dilute solutions: the first by measuring the increase in the boiling point of a pure solvent, and the second by measuring the decrease in the crystallization temperature of a pure solvent. Since polycyclic aromatic and naphthenoaromatic hydrocarbons tend to associate, the molecular weight is determined at different concentrations of oil in the solvent and the true molecular weight is calculated by extrapolation to zero concentration.

The refractive index characterizes the change in the speed of light when passing from one medium to another and is measured by the ratio of the sine of the angle of incidence of light to the sine of the angle of its refraction. The refractive index depends on the wavelength of light and temperature and, for given values ​​of these parameters, is a characteristic of the substance. Similar to density, the refractive index value decreases as the cleaning gets deeper. With similar fractional compositions and viscosities of oils, the refractive index satisfactorily characterizes the content of aromatic hydrocarbons.

Viscosity characterizes the property of a liquid to resist the movement of one part of the liquid relative to another (Figure 1).

Usually they use the concept of kinematic viscosity, which is the ratio of dynamic viscosity to density; its unit is taken in the SI system as 1 m 2 /s.

Viscosity is sometimes expressed in other units - degrees Engler. Abroad they use Saybolt and Redwood degrees.

In practice, it is often important to know the viscosity of the oil at low temperatures, which is difficult to determine experimentally. For this purpose, the viscosity is determined at two positive temperatures, their values ​​are connected with a straight line on the nomogram and extrapolated to the desired temperature (Figure 1).

Picture 1

It should be taken into account that the nomogram is based on the assumption that in the accepted temperature range the oil behaves as a Newtonian fluid.

At temperatures close to the pour point, a viscosity anomaly appears. The nomogram can be used up to temperatures 10-15 °C above the pour point.

In practice, the Dean and Davis viscosity index has found widespread use. These authors proposed to compare the viscosity of the test oil with the viscosity of oil distillates obtained from American oils of the Pennsylvania and Gulf of Mexico. The viscosity index of the first oil is taken as 100, and the second as 0.

All oils at 98.9 °C should have the same viscosity.

The density, refractive index and viscosity of oils depend on the chemical and primarily hydrocarbon composition of oils with a similar fractional composition.

The flash point of transformer oils is determined in a closed crucible in a Martin-Pensky apparatus.

The flash point is the temperature at which balls of oil heated under standard conditions burst into flame when a flame is applied to them.

The flash point for conventional commercial oils ranges from 130 to 170, and for Arctic oil - from 90 to 115 ° C and depends on the fractional composition, the presence of relatively low-boiling fractions and, to a lesser extent, on the chemical composition.

The flash points of oils depend on the elasticity of their saturated vapors. The lower the vapor pressure and the higher the flash point, the better it is to degas and dry the oil before filling it into high-voltage equipment. The minimum flash point of oils is regulated not so much for fire safety reasons, but from the point of view of the possibility of their deep degassing.

In relation to fire safety, the auto-ignition temperature plays an important role; This is the temperature at which oil, in the presence of air, ignites spontaneously without the application of a flame. For transformer oils this temperature is about 350-400 °C.

For domestic transformer oils, the saturated vapor pressure at 60 °C ranges from 8 to 0.4 Pa. Foreign oils, as a rule, have a lower vapor pressure and range from 1.3 to 0.07 Pa.

The volumetric weight of oil for transformers is not a fixed nameplate value. It is clear that this oil, like any other liquid, will have different volumes when placed in different vessels. Therefore, let's talk about the passport characteristics, such as the volumetric weight of transformer oil.

Determination of volumetric weight

Let's start with a definition. The volumetric weight of oil is the ratio of its weight at a temperature of +20 ºС to the weight of water occupying the same volume, but at a temperature of +4 ºС.

Indicators of the standard volumetric weight of oil for transformers

This indicator is not standardized. At a temperature of +20 ºС for transformer oil it is 0.856-0.886. If you heat, the value of the volumetric weight will decrease, and when cooled, on the contrary, it will increase.

Change factor

To determine the volumetric weight of oil at a temperature that differs from +20 ºС, when it increases, you need to subtract, and when it decreases, add the coefficient of change in volumetric weight for each degree. Typically, for electrical insulating oils, the numerical value of this indicator is 0.0007 per 1 ºС.

GOST

You can also use a special technique set out in GOST-3900-47 to determine the volumetric weight. There is also a table containing corrections for temperatures not equal to +20 ºС.

Instruments for determining the volumetric weight of transformer oil

In practice, the simplest way to determine volumetric gravity is to use a hydrometer (oil densimeter). A portion of the test oil is taken into a glass cylinder, and then a hydrometer is placed there. The count is taken along the upper edge of the meniscus.

Effect of temperatures

If the oil temperature is changed by +100 ºС, for example, from -35 ºС to +65 ºС, then its volume will change by approximately 7%. Considering the fact that during operation the temperature can vary over a wider range, the volume of the conservator should be selected at the level of 9-10% of the oil volume.

6. The validity period was lifted according to Protocol No. 2-92 of the Interstate Council for Standardization, Metrology and Certification (IUS 2-93)

7. EDITION (June 2011) with Amendments No. 1, 2, 3, approved in March 1982, March 1985, March 1989 (IUS 7-82, 6-85, 6-88), Amendment (IUS 6-2005)


This standard applies to transformer oils of sulfuric acid and selective purification, produced from low-sulfur oils and used for filling transformers, oil switches and other high-voltage equipment as the main electrical insulating material.

1. BRANDS

1. BRANDS

The following brands of transformer oils are installed:

TK - without additive (manufactured according to special orders for general technical purposes), it is not allowed to be used for filling transformers;

T-750 - with the addition of (0.4±0.1)% antioxidant additive 2,6 ditertiary butyl paracresol;

T-1500 - with the addition of at least 0.4% antioxidant additive 2,6 ditertiary butyl paracresol;

PT is a promising oil.

(Changed edition, Amendment No. 1, 3).

2. TECHNICAL REQUIREMENTS

2.1. Transformer oils must be manufactured in accordance with the requirements of this standard, from raw materials and using technology that were used in the manufacture of oil samples that have been tested with positive results and approved for use in the prescribed manner.

2.2. In terms of physical and chemical parameters, transformer oils must comply with the requirements and standards specified in the table.

(Changed edition, Amendment No. 2, 3, Amendment).

3. SAFETY REQUIREMENTS

3.1. Transformer oils are low-hazard products and, in terms of the degree of impact on the human body, belong to the 4th hazard class in accordance with GOST 12.1.007.

3.2. Transformer oils are, in accordance with GOST 12.1.044, flammable liquids with a flash point of 135 °C.

3.3. The room in which work with oil is carried out must be equipped with supply and exhaust ventilation.

3.4. The maximum permissible concentration of oil hydrocarbon vapors in the air of the working area is 300 mg/m in accordance with GOST 12.1.005.

3.5. When working with transformer oils, personal protective equipment must be used in accordance with standard rules approved in accordance with the established procedure.

3.6. When oils catch fire, use the following fire extinguishing agents: sprayed water, foam; for volumetric extinguishing - carbon dioxide, SRC composition, composition 3.5, steam.

Section 3. (Changed edition, Amendment No. 3).

4. ACCEPTANCE RULES

4.1. Transformer oil is accepted in batches. A batch is considered to be any quantity of oil produced during a technological process, homogeneous in terms of quality, accompanied by one quality document containing data in accordance with GOST 1510.

(Changed edition, Amendment No. 3).

4.2. The volume of samples is in accordance with GOST 2517.

4.3. If unsatisfactory test results are obtained for at least one of the indicators, repeated tests are carried out on a newly selected sample from the same sample.

The results of repeated tests apply to the entire batch.

(Changed edition, Amendment No. 3).

5. TEST METHODS

5.1. Transformer oil samples are taken according to GOST 2517.

For a combined sample, take 3 dm of each brand of oil.

(Changed edition, Amendment No. 1).

5.2. The sodium sample for T-750 and T-1500 grade oils is determined in a 20 mm cuvette, for TK grade oil - in a 10 mm cuvette.

5.3. The transparency of transformer oils is determined in a glass test tube with a diameter of 30-40 mm. Oil at a temperature of 5 °C should be transparent in transmitted light.

5.4. The sediment index and acid number for TK grade oil are determined according to GOST 981 under the following conditions:

temperature - 120 °C,



oxygen consumption - 200 cm/min,

The duration of oxidation when determining sediment and acid number is 14 hours.

The indicator of low molecular weight volatile acids can be determined under the following conditions:

temperature - 120 °C,

catalyst - balls with a diameter of (5±1) mm, one made of low-carbon steel, one made of copper grade M0k or M1k according to GOST 859;

air flow - 50 cm/min;

oxidation duration - 6 hours.

The stability against oxidation of T-750 and T-1500 grade oils is determined according to GOST 981 under the following conditions:

temperature for T-750 oil - 130 °C, for T-1500 oil - 135 °C,

catalyst - copper plate,

oxygen consumption - 50 cm/min,



The oxidation stability of promising hydrocracking oil is determined according to GOST 981 under the following conditions:

temperature - 145 °C,

catalyst - copper plate;

oxygen consumption - 50 cm/min;

oxidation duration - 30 hours.

(Changed edition, Amendment No. 1, 2, 3).

5.5. The dielectric loss tangent of transformer oils is determined without preparation or after preparation in one of the following ways:

a) 100 cm of oil is kept for 30 minutes at 50 °C with a residual pressure of 666.6 Pa (5 mm Hg) in a vessel with a free surface equal to 100 cm;

b) the oil is kept in a crystallizer placed in a desiccator with calcined calcium chloride for at least 12 hours with a layer thickness of no more than 10 mm.

In case of disagreements arising when assessing product quality, oil preparation before determining the dielectric loss tangent is carried out according to subparagraph a.

To determine the dielectric loss tangent, electrodes made of stainless steel grade 12Х18Н9Т or 12Х18Н10Т according to GOST 5632 are used. When manufacturing electrodes from copper according to GOST 859 and brass according to GOST 17711, the working surfaces of the electrodes must be coated with nickel, chromium or silver. The determination is carried out at an electric field strength of 1 kV/mm.

6. PACKAGING, LABELING, TRANSPORTATION AND STORAGE

6.1. Packaging, labeling, transportation and storage of transformer oils - in accordance with GOST 1510.

6.2. The document certifying the quality of transformer oil of the T-750 and T-1500 brands of the highest category and the container must display the state Quality Mark.

7. MANUFACTURER WARRANTY

7.1. The manufacturer guarantees that the quality of transformer oil meets the requirements of this standard subject to the conditions of transportation and storage.

7.2. The guaranteed shelf life of transformer oils is five years from the date of manufacture.

(Changed edition, Amendment No. 2).



Electronic document text
prepared by Kodeks JSC and verified against:
official publication
Oil and petroleum products. Oils.

Technical conditions. Collection of GOSTs. -

M.: Standartinform, 2011

In the same connection is the experimental fact that with a decrease in the viscosity of transformer oil when it is heated, the absorption coefficient does not decrease (as it should be for waves of small amplitude), but increases.  

As for the change in oil viscosity at low temperatures1, as follows from table. 11, borrowed from the same work, a sharp increase in the viscosity of transformer oil is observed already at temperatures below minus 30 C, and for turbine oil at a temperature of minus 5 C.  

For use in power transformers in the USSR, they mainly use Sovtol-10, which is a mixture of 90% pentachlorobiphenyl and 10% trichlorobenzene, which has a viscosity in the operating temperature range close to the viscosity of transformer oil. However, in terms of its viscosity-temperature properties, Sovtol-10 is significantly inferior to hexol, which is a mixture of 20% pentachlorobiphenyl and 80% hexachlorobutadiene. Gek-sol does not harden at temperatures down to - 60 C and is less susceptible to the influence of pollution.  

Two series of experiments were carried out. The viscosity of transformer oil was reduced by adding a solvent - kerosene - to it and dissolving natural gas in it.  

The viscosity of transformer oil is strictly standardized. Transformer oil supplied to enterprises is carefully dried in special installations and filtered many times. The breakdown voltage of the oil before pouring into the transformer must be at least 50 kV with a distance between two electrodes in a standard punch of 2 5 mm.  

In most cases, dry transformer oil (GOST 982 - 56), which has good electrical insulating properties, is used for this purpose. The viscosity of transformer oil is low, as a result of which its convection and circulation provide good cooling of the equipment, which is especially important for devices with elements that heat up during operation. The oil also protects equipment from atmospheric influences and from the harmful effects of chemically aggressive environments.  

The main advantage of transformer oil is its high insulating properties and the ability to protect the cooled path from corrosion. However, the viscosity of transformer oil is much higher than the viscosity of water. Therefore, to create oil circulation comparable in efficiency to water circulation, larger pipeline diameters and higher pressure are required. The oil pressure in the pipeline is limited to 3 - 4 kgf/cm2, since due to the good wettability of metal surfaces, at high pressures it can leak through minor leaks, which almost always occur in pipeline joints.  

In technical standards, the value v20 is indicated as one of the parameters characterizing this oil, but in Fig. Therefore, we will determine the viscosity of purified transformer oil at 20 C approximately using, for example, Gross’ formula (I, 56).  

This can be especially valuable for power limit transformers that would otherwise be non-transportable. It should be noted that the viscosity of transformer oil increases with decreasing temperature, so the heat transfer coefficient from the windings to the oil will be lower than in conventional oil transformer systems.  

If the stator cavity is filled with transformer oil, then during start-up in winter it is necessary to create a minimum load or, if this is permissible, start in idle mode and continue operating the electric motor in this mode to warm up the entire volume of oil to 15 - 20 C without supplying cooling fluid into the cooling system. This is necessary because the viscosity of transformer oil at low temperatures is high and its circulation throughout the entire circuit will be difficult, which can lead to local overheating and charring of the winding insulation even when the oil temperature at the measuring points has not yet reached the limit values .  

The operation of electric motors, the stator cavity of which is filled with transformer oil or water cooling is used to remove heat, in winter in open areas or in unheated rooms has a number of distinctive features. This is because at low temperatures the viscosity of transformer oil increases and water in the cooling system can freeze if proper precautions are not taken.  

A decrease in viscosity at a given flash point is achieved by narrowing the fractional composition; the implementation of this measure is limited, since this reduces the oil yield. In recent years, there has been a tendency abroad to reduce the viscosity of transformer oils, even with a slight decrease in the flash point.  

Transformer oil is a product obtained from petroleum. It is used as an electrical insulating material, a heat sink and arc extinguishing medium, and a medium that protects solid insulation from air and moisture. As you can see, the list of tasks performed is quite wide, which puts forward certain requirements for the properties of transformer oils. In this article I would like to talk about what the viscosity of transformer oil is.

Among other properties of electrical insulating oils, viscosity is perhaps one of the most important. Fresh oil that is just poured into the transformer should have as low a viscosity as possible. This will help improve heat removal from the windings.

A similar situation is observed in oil switches. Their oil must have high mobility and low viscosity so that the resistance provided to moving parts is minimal. Modern switches put forward new requirements for the viscosity of oils and the dependence of its increase on the decrease in temperature.

What is oil viscosity?

Viscosity is one of the most important properties of transformer oils, which is associated with its great influence on heat exchange processes occurring in oil-filled equipment.

When performing engineering calculations, the concepts of specific, kinematic and dynamic viscosity are used. As with many things, compromises must be made when choosing an oil for electrical equipment. The thing is that a material with high viscosity has a good effect on electrical insulating properties, and low viscosity reduces the cooling ability. Therefore, in practice, the optimal option is selected, which is able to ensure good performance of both the first and second functions.

Since the operating conditions of power transformers are quite severe and can be characterized by elevated temperatures, it is worth taking into account the change in viscosity when heated. An increase in temperature leads to a decrease in viscosity and vice versa.

Typically, in the reference literature you can find several viscosity values ​​for transformer oil, indicated for a specific temperature. Using well-known mathematical methods (interpolation, extrapolation, etc.) it is easy to find the viscosity value at the temperature of interest, even if it is not indicated in the reference book. For example, the average kinematic viscosity for transformer oil is (28‑30)∙10 -6 m 2 /s.

Conditional and kinematic viscosity of transformer oil

A parameter like nominal viscosity, are determined using a special device - an Engler viscometer, according to the method outlined in GOST 6558-52. At the same time, they look at the so-called water number of the viscometer: i.e. flow of 200 cm of distilled water at 20 ºС. It should not be less than 50 and more than 52.

Kinematic viscosity determined using a capillary viscometer (Pinkevich viscometer), which has the form of a Y-shaped tube. The measurement technique is set out in GOST 33-82.

In practice, when choosing the viscosity of oils, it is necessary to seek a compromise, since on the one hand, its high value has a good effect on electrical insulating properties, but worsens the cooling ability and increases the resistance of moving parts of mechanisms. Low viscosity has the opposite effect.

As a rule, different types of transformer oils have different viscosities. This indicator depends significantly on temperature(if the oil is heated, its viscosity decreases), therefore, in the reference literature, in most cases, several values ​​​​of this indicator are indicated at different temperatures.

For example, when positive operating temperatures from 50 ºС to 90 ºС the viscosity of oils of different origins can differ by approximately two times. For various oils at positive temperatures, the temperature gradient of viscosity does not exceed 1 mm 2 /s per 1 ºC.

When negative temperatures The viscosity of different types of oils can increase very unevenly. Judge for yourself: in the range of -20 ºС… -30 ºС the temperature gradient of viscosity is 60-70, -30 ºС… -40 ºС – 90-370, -40 ºС… -50 ºС – 800-6000, and in the range -50 ºС ... -60 ºС can reach 50,000 mm 2 /s at 1 ºС and higher.

If a change in the viscosity of transformer oils occurs at low temperatures, then in this case it is necessary to take into account such a phenomenon as anomaly viscosity Also, a discount on high viscosity values ​​should be made if a powerful transformer with circulation cooling is put into operation. In such devices, the oil is exposed to low temperatures for a long time.

In devices such as oil switches or contactors for regulating voltage under load of transformers, performance also directly depends on viscosity.

Viscosity measurement of transformer oils

Determination of the conditional viscosity of transformer oils is carried out using special instruments - Engler viscometers. They consist of a brass and metal vessel, a calibrated tube, a stopper and index pins.

Oil viscosity in Engler degrees is the time required for the flow of 200 milliliters of oil heated to a temperature of 50ºC, divided by the time for the same volume of distilled water to flow, but at a temperature of 20ºC.

To find dynamic and kinematic viscosity, special empirical formulas are used that take into account the force acting on a solid ball in oil, its radius, speed of movement, radius and height of the vessel. Kinematic viscosity is obtained by dividing the known dynamic viscosity by the density of the transformer oil.

In addition to Engler instruments, other viscometers can also be used to measure conditional viscosity: rotational, ball, electrorotational, capillary and plastic viscometers.

To maintain the optimal numerical value of the viscosity of transformer oil throughout its entire service life, it is necessary to use special equipment. The thing is that during the operation of power transformers, oils are exposed to a number of unfavorable factors: sunlight, high temperatures, air oxygen, mechanical impurities, etc. The combination of these factors leads to a deterioration in the operating parameters of oils and their deviation from the standardized values. First of all, we are talking about breakdown voltage, acid number, dielectric loss tengent, and flash point. Viscosity is no exception.

Therefore, in order to maintain all operational parameters of transformer oil at the level of standardized values, it is necessary to carry out certain measures: cleaning, drying and regeneration.

Company GlobeCore offers a wide selection of equipment designed to work with transformer oils. Application of technology GlobeCore allows not only to maintain the parameters of transformer oils at the proper level, but also to restore them in case of deterioration.

Installations for cleaning, drying and regeneration of transformer oils from the company GlobeCore is an energy-efficient and environmentally friendly solution to the problem of maintaining and deteriorating the quality characteristics of transformer oils! To ensure reliable operation of your oil-filled equipment, simply contact the specialists of our company and, with their help, select the installation of the required performance.